Diazotization reactions have been known for over a century and have been applied in a large scale in the manufacture of organic products, particularly in the manufacture of dyes, medicines, or intermediate products thereof. In particular, diazotizations and couplings are the chemical foundation of the azo dye industry. Production of azo dyes on an industrial scale is usually carried out batchwise in an aqueous medium. A sodium nitrite (NaNO.sub.2) solution and a mineral acid, such as muriatic acid or dilute sulfuric acid, are used for diazotization in a majority of cases. In this regard, it often is advantageous to add sodium nitrite to the reaction below liquid level via a dip pipe.
It has long been recognized that diazotization is a complex reaction involving the formation of several active nitrosating species, as well as nitrogen oxides. However, the formation of unacceptable levels of nitrogen oxides during the diazotization may require that the addition of sodium nitrite be stopped to prevent discharge of the oxides to the atmosphere. High concentrations of nitrogen oxides in the vent system also may lead to fires or explosions if any organic material has been deposited in the system during previous charging operations.
Prior to the present invention, dip pipes with anti-syphon holes used for the subsurface addition of sodium nitrite in diazotization reactors have not completely assured the prevention of reactor backflow. Furthermore, conventional dip pipes tend to spray sodium nitrite through the anti-syphon holes and thereby promote the formation of nitrogen oxides. In this instance, a flow reversal, consequential cross contamination, and a possible explosion can not be ruled out.
Accordingly, there continues to be a need for an improved dip pipe which can be used for the subsurface addition of a sodium nitrite solution in a diazotization process without promoting the formation of nitrogen oxides.